Brainstem

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Brainstem


The brainstem is the stalk-like part of the brain which connects the spinal cord with the forebrain. From below upwards it consists of three parts: medulla oblongata, pons, and midbrain. The midbrain is continuous above with the cerebral hemispheres and the medulla oblongata is continuous below with spinal cord. Brainstem is located in the posterior cranial fossa. Its ventral surface lies on the clivus. Posteriorly, the pons and medulla are separated from the cerebellum by the cavity of the fourth ventricle.


The brainstem, like spinal cord consists of nerve fibres and nerve cells. Most of the nerve fibres in the brainstem are arranged longitudinally in the form of tracts as in the spinal cord. But the nerve cells of the brainstem do not form a single central mass of grey matter as in the spinal cord; rather most of them are aggregated to form the well defined nuclei. In addition to the well defined tracts and nuclei, the brainstem consists of diffuse system of nerve cells and nerve fibres called reticular formation. The central canal of spinal cord continues upwards in the lower part of the brainstem and then it widens and moves dorsally to form the fourth ventricle.


The brainstem serves the following four major functions:





Medulla Oblongata


The medulla oblongata is the direct upward continuation of the spinal cord, extending from foramen magnum to the lower border of the pons. It forms the lowest part of the brainstem and lies almost vertically in the anterior part of the posterior cranial fossa between the clivus (superior surface of the basi-occiput) in front and the vallecula of the cerebellum behind.


The medulla is shaped like a truncated cone (bulb-like) hence its alternative name ‘bulb’ (cf. bulbar paralysis).


Medulla oblongata measures about 3 cm in length, 2 cm in breadth (at the widest part) and 1.25 cm in thickness).


The medulla contains vital centres which are essential for life. These are: (a) cardiac centre, (b) vasomotor centre, and (c) respiratory centre.


Medulla provides attachment to last four cranial nerves.


The lower part of the medulla like the spinal cord contains the central canal. In the upper part of the medulla this canal widens and moves dorsally to form the lower part of the fourth ventricle. Thus, the medulla is divided into a lower closed part and an upper open part.


An overview of structural components of medulla and their functions is provided in Table 8.1.






External Features


The medulla is divided into right and left symmetrical halves by anterior median fissure and posterior median sulcus (Fig. 8.1).



The anterior median fissure is continuous below with the corresponding fissure on the spinal cord, and above it ends into a small triangular depression called foramen caecum, at the lower border of the pons. It is interrupted in its lower part by the bundles of fibres crossing obliquely from one side to the other, the decussation of pyramids.


The posterior median sulcus continues below with the corresponding sulcus of the cord and is present only in the lower half of the medulla. Above its lips diverge to form the boundaries of a triangular area, the lower part of the floor of fourth ventricle.


Each half of the medulla is marked by two sulci— anterolateral and posterolateral, which are direct upward continuations of the corresponding sulci of the cord.


The anterolateral sulcus extends along the lateral border of the pyramid and along it emerges the rootlets of the hypoglossal (XII cranial) nerve.


The posterolateral sulcus lies between the olive and the inferior cerebellar peduncle and along it emerges the rootlets of glossopharyngeal (IX), vagus (X) and accessory (XI) cranial nerves.


The anterolateral and posterolateral sulci with nerve roots divide the surface of each half of the medulla oblon-gata into anterior, posterior and lateral regions like that in the spinal cord.



Features on the anterior (ventral) aspect of medulla (Fig. 8.1)


The ventral aspect of medulla presents following features:



• Pyramids. These are two elongated elevations, one on either side of anterior median fissure and are produced by the corticospinal (pyramidal) fibres. Most of these fibres about (75%) cross to the opposite side (pyramidal decussation) in the lower part of medulla and then descend as lateral corticospinal tract in the lateral white column of the spinal cord. About 20% of uncrossed fibres run downwards as anterior corticospinal tract in the anterior white column of the spinal cord; the remaining 5% run downwards along with uncrossed fibres in the lateral white column of the spinal cord (Fig. 8.2).



• Olives. These are oval elevations, posterolateral to the pyramids and are produced by an underlying mass of grey matter called inferior olivary nucleus.


• Rootlets of the hypoglossal nerve. These emerge from the anterolateral sulcus between the pyramid and the olive.


• Inferior cerebellar peduncles. These are thick bundles of fibres lying posterolateral to the olive, and attach the medulla with the cerebellum.


• Rootlets of the IXth, Xth, and XIth (cranial part) cranial nerves. These emerge through the posterolateral sulcus separating the olive from the inferior cerebellar peduncle.



Features on the Posterior (Dorsal) Aspect of Medulla (Fig. 8.3)


The dorsal aspect the medulla is well demarcated into lower closed and upper open parts.






Internal Structure


As in the spinal cord, in the medulla also, the white matter surrounds the central mass of grey matter but in the latter, the grey matter is broken up into nuclei that are separated by nerve fibres. The internal structure of medulla is well appreciated by examining its transverse sections at the following four levels:




Transverse section of medulla at the level of pyramidal decussation (the great motor decussation)


The section at this level passes through the inferior half of the medulla, and closely resembles to that of spinal cord. However, following important features are observed at this level (Fig. 8.4):




• The nucleus gracilis and nucleus cuneatus appear as the narrow strip like projections from the posterior aspect of the central grey matter.


• The apex of posterior horn gets swollen up to form the nucleus of spinal tract of trigeminal nerve. It is an upward continuation of the substantia gelatinosa in the posterior grey column of the spinal cord.


• The spinal tract of trigeminal nerve is a bundle of fibres which caps over the nucleus of spinal tract of trigeminal nerve.


• Decussation of pyramidal tracts forms the most important feature of medulla at this level. About 75% fibres of pyramidal tract run backwards and laterally across the midline to reach the lateral white column of the opposite side of the spinal cord, where they run downwards as the lateral corticospinal tract. In doing so the anterior horns are detached from the central grey matter.


• Each detached anterior horn divides to form the spinal nucleus of accessory nerve and supraspinal nucleus of first cervical nerve. The nucleus of accessory nerve extends downwards up to fifth cervical spinal segment. The supraspinal nucleus gives off the efferent fibres of the first cervical nerve, and is continuous above with the nucleus of the hypoglossal nerve.


• Appearance of diffuse zone containing a network of fibres and scattered nerve cells within it the lateral white column adjacent to nucleus of spinal tract of trigeminal nerve is called reticular formation.



Transverse section of medulla at the level of sensory decussation


This section passes through the middle of medulla and displays following features (Fig. 8.5):




• The nucleus gracilis and nucleus cuneatus become more pronounced and are separated from the central grey matter. The fibres of fasciculus gracilis and fasciculus cuneatus occupy the broad posterior white column and terminate in these nuclei.


• The internal arcuate fibres arising from the cells of gracile and cuneate nuclei (second order sensory neurons conducting sensations of discriminative touch, position and vibration) course forwards and medially around the central grey matter and decussate with corresponding fibres of opposite side in the median plane (sensory decussation) and then turn upwards to ascend as the medial lemniscus on the opposite side close to the median plane. In this decussation the gracile fibres are medial to that of cuneate fibres.


• The internal arcuate fibres cut off the spinal nucleus and tract of trigeminal nerve from the central grey matter.


As the fibres from nucleus gracilis and nucleus cuneatus pass forwards and medially they intercross so that the most medial fibres (from the feet and leg) come to lie anteriorly in the medial lemniscus. In the medial lemniscus the body is represented with the head posteriorly and the feet anteriorly. The fibres of lemniscus relay into the corresponding thalamus.




• Immediately dorsolateral to the cuneate nucleus lies the accessory cuneate nucleus which receives the more lateral fibres (derived from the cervical segments of the cord) of the fasciculus cuneatus and gives rise to posterior external arcuate fibres conveying proprioceptive impulses to the cerebellum of the same side through inferior cerebellar peduncle.


• The separated spinal nucleus and tract of trigeminal nerve lies ventrolateral to the cuneate nucleus.


• The lower part of inferior olivary nucleus is seen.


• The pyramids lie on either side of the anterior median fissure.


• The central grey matter contains: (a) hypoglossal nucleus, (b) dorsal nucleus of vagus, and (c) nucleus of tractus solitarius. The hypoglossal nucleus occupies the ventro-medial position close to the midline in the central grey matter. The dorsal nucleus of vagus lies dorsolateral to the hypoglossal nucleus and nucleus of tractus soli-tarius lies just dorsolateral to the dorsal nucleus of vagus.


• Medial longitudinal bundle lies posterior to the medial lemniscus. It is small compact tract of nerve fibres which interconnect the IIIrd, IVth, VIth, VIIIth and spinal nucleus of XIth cranial nerve nuclei.


• Spinocerebellar and lateral spinothalamic tracts lie in the anterolateral area of lateral white column.


• Lateral and anterior spinothalamic tracts are very close to each other and collectively form spinal lemniscus.



Transverse section of medulla at the level of olives


Transverse section passes across the floor of the fourth ventricle and through the middle of olives and presents following features (Fig. 8.6):




• The central grey matter is spread over the floor of the fourth ventricle and contains the nuclei of several cranial nerves. From medial to lateral these are: hypoglossal nucleus, nucleus intercalatus, dorsal nucleus of vagus and vestibular nuclei (inferior and medial).


• The nucleus of tractus solitarius lies ventral to vestibular nuclei.


• The nucleus ambiguus lies deep within the reticular formation and gives origin to the motor fibres of IXth, Xth and XIth cranial nerves.


• On either side of midline (paramedian region), from dorsal to ventral lie: medial longitudinal fasciculus, tectospinal tract, medial lemniscus, and pyramidal (corticospinal) tract.


• The arcuate nuclei, thought to be inferiorly displaced pontine nuclei are situated on the anteromedial aspect of the pyramids. They receive fibres from the cerebral cortex and send efferent fibres to the cerebellum of the opposite side through the anterior external arcuate fibres.


• Laterally, from dorsal to ventral lie two prominent structures: (a) inferior cerebellar peduncle, and (b) inferior olivary nucleus.


    





Blood Supply of the Medulla


The medulla is supplied by the following arteries:




Clinical Correlation



Vascular Disorders of Medulla Oblongata


The common vascular lesions involving the medulla are the thrombosis of posterior inferior cerebellar and vertebral arteries leading to lateral and medial medullary syndromes respectively.




The dorsolateral part of the medulla is supplied by the posterior inferior cerebellar artery, which is usually the branch of the vertebral artery. This artery also supplies the inferior surface of the cerebellum.


Thrombosis of posterior inferior cerebellar artery therefore, affects a wedge-shaped area on the dorsolateral aspect of medulla (Fig. 8.7) and the inferior surface of the cerebellum and produces following signs and symptoms.







The paramedian region of the medulla is supplied by the branches of vertebral artery. The vascular involvement (ischaemia) of this region produces following signs and symptoms:


Jan 2, 2017 | Posted by in NEUROLOGY | Comments Off on Brainstem
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